1. Field of the Invention.
The present invention is directed to solid ink-jet ink compositions suitable for use in color transparencies. Specifically, the present invention is directed to a substantially homogeneous solid ink-jet ink composition comprising a carrier and a non-volatile driver. Typically, these compositions also will contain a colorant and, in a preferred embodiment, such solid ink-jet ink compositions further contain a cosolvent. The present invention is also directed to methods for preparing colored transparencies.
2. References Cited.
The following references are cited in this application as superscript numbers at the relevant portion of the application:
1. Vaught, U.S. Pat. No. 4,490,731 PA0 2. Lloyd et al., Ink Jet Printing, pp. 311-369 (1988) PA0 3. Vaught et al., U.S. Pat. No. 4,490,728 PA0 4. Gerstenmaier, U.S. Pat. No. 4,745,420; PA0 5. Creagh et al., U.S. Pat. No. 4,801,473; and PA0 6. Van Brimer et al., European Pat. Appl. Pub. No. 308 117A
3. State of the Art.
Solid ink-jet ink compositions have been successfully employed in thermal ink-jet printing utilizing thermally induced vapor bubbles as a driving force to eject ink drops out of the nozzle of the jet printer and onto the substrate. See, for example, Vaught 1 , Lloyd et al. 2, and Vaught et al. 3 which are incorporated herein by reference in their entirety.
Such solid ink-jet ink compositions preferably contain as separate ingredients, a carrier and a driver and can additionally contain a colorant as well as any optional additives. The carrier is an organic material which carries any colorant(s) and which is a solid at room temperature (i.e., about 25.degree. C.) and a liquid at the operating temperature of the print head (e.g., about 90.degree. C. to 120.degree. C.). The driver is a bubble-forming substance which can provide enough driving force to eject ink drops from the print head or nozzle. The colorant can be a dye or pigment, which is compatible (soluble or dispersible) in the carrier and which produces the visible printed images on the substrate.
In practice, a portion of the solid ink-jet ink composition in the print head of the thermal ink-jet printer is first liquified and then a portion of driver found in the so liquified ink is vaporized so as to generate a bubble which is used to force a drop of the liquified ink from the injection nozzle of the print head onto the substrate.
While such ink-jet ink compositions are successfully employed on substrates such as paper, there is a problem when such compositions are employed for preparing colored transparencies for projecting colored images via an overhead projector. Specifically, color printing on transparent substrates is complicated by the fact that current thermal ink-jet printers have difficulty in generating colored transparencies having good color clarification of the projected image and, in fact, produce colors having grayish shades and poor color definition.
In regard to the above, color projection via an over-head projector would be theoretically achieved by passing visible light through a transparent substrate having appropriately deposited thereon colored but otherwise translucent ink compositions. A portion of the light penetrating the transparency would then be absorbed by the ink composition in that portion of the spectrum corresponding to the color of the ink with the remainder of the light passing unaffected through the transparency. This non-absorbed light is then projected by the overhead projector onto the screen or other viewing device so as to provide a colored projection (i.e., the color of the projection corresponds to the color of the ink compositions).
However, while theoretically sound, there is a continuing problem with prior art ink-jet ink compositions suitable for use with thermal ink-jet printers which eject ink drops by bubble formation. Specifically, these compositions do not provide excellent colored transparencies because as heretofore formulated, these compositions do not permit the non-absorbed light to pass through unaffected but instead the composition causes excessive scattering of the non-absorbed light as it passes through the ink composition. This excessive scattering leads to the difficulties in correlating the projected color with the color found on the transparency and a grayish image is projected.
It has now been found that such excessive scattering is associated with difficulties in forming a miscible multicomponent ink-jet ink composition which is stable from room temperature (typically the storage temperature of the ink composition) to about 90.degree. C. to 120.degree. C. (typically the operating temperature range of a print head). Specifically, if the ink composition is not stable and miscible, then the composition will have or will develop regions of different physical and/or chemical environments therein which, in turn, leads to excessive light scattering because each beam of light passing through a different portion of the supposedly same ink composition can encounter a different physical and/or chemical environment compared to a different beam of light.
Without being limited to any theory, this problem with excessive light scattering is believed to be primarily associated with the use of the driver in the solid ink-jet ink composition. Specifically, because the carrier and the colorant are typically hydrophobic, high boiling point materials, these materials will generally be soluble in each other and, when mixed at elevated temperatures in a liquid phase, will provide for a miscible multi-component composition which is stable from room temperature (typically the storage temperature of the ink composition) to about 90.degree. C. to 120.degree. C. (typically the operating temperature range of a print head). In turn, a stable miscible composition obviates the problem with light scattering because with such a composition, light beams penetrating different portions of such a composition will encounter a similar physical and/or chemical environment.
On the other hand, the driver is typically a component which is more hydrophilic and more volatile (e.g., has a lower boiling point) than the either the carrier or the colorant. In fact, most drivers heretofore employed in ink-jet ink compositions are typically liquids at room temperature. Accordingly, formulating a miscible multi-component solution which is stable from room temperature (typically the storage temperature of the ink composition) to about 90.degree. C. to 120.degree. C. (typically the operating temperature range of a print head) is unlikely if a volatile driver is employed. Such instability can lead to regions in the ink composition which are amorphous and to regions which are crystalline. In such a case, light beams penetrating different parts of such an ink composition will likely encounter different physical and/or chemical environments which, as noted above, leads to increased levels of light scattering.
Moreover, even when a stable composition is formed, if a volatile driver is employed in such a composition, over time a portion of the driver can evaporate from the composition. Because the rate of evaporation from the surface of the composition as compared to the interior of the composition is non-uniform, such non-uniform evaporation will result in regions of different physical and/or chemical environments within the composition, which as noted above, leads to increased levels of light scattering.
As a further complication, the use of a non-volatile driver can nevertheless result in excessive light scattering. Specifically, non-volatile drivers are non-volatile at room temperature in the ink composition but which possess sufficient volatility so as to effectively form vapor bubbles at the operating conditions of the print head. Such non-volatile drivers are nevertheless still more hydrophilic than either the carrier or the colorant and accordingly, these drivers will typically have limited solubility with such other components. Because of its low solubility in the ink composition, solid ink-jet ink compositions containing such drivers can result, over a period of time, in some phase separation which results in the formation of driver granules (including driver crystals), when the driver is a solid, and in the formation of globules or vesicles, when the driver is a liquid. Such granular or globular formation can lead to regions in the ink composition which have different physical and/or chemical environments, particularly if the granule or globule size is sufficiently large, which can lead to increased levels of light scattering.
In view of the above, while it would be particularly desirable for solid ink-jet ink compositions containing a driver to produce colored transparencies with excellent color projection, such was not possible with driver containing solid ink-jet ink compositions heretofore known.